Highly diastereoselective synthesis of chiral aminophenolate zinc complexes and isoselective polymerization of rac-lactide

Abstract

An enantiopure zinc complex supported by an aminophenolate ligand with multiple stereogenic centers has been diastereoselectively synthesized via the variation of the ortho-substituent of a phenoxy moiety and the N-alkyl group of a chiral pyrrolidinyl ring in the ligand framework, which displays high isoselectivity in the polymerization of rac-lactide.

Chiral (1,2)‐Diphenylethylene‐Salen Complexes of Triel Metals: Coordination Patterns and Mechanistic Considerations in the Isoselective ROP of Lactide

“…4 Aluminium has actually proved in a class of its own, as Al complexes supported by dianionic tetradentate salen,3–5 salan6and structurally related salalen and dialkoxy‐diimino7 ligands reliably enable the production of genuinely isotactic PLA (0.80< P m <1.00; see below) 8. Otherwise, mitigated success in the isoselective ROP of LA has sporadically been achieved in the past 15 years with complexes of various metals (Ti, In, Y, Zn)8, 9, 10 typically supported by a monophenolate ligand, affording PLA with a more moderate isotactic bias ( P m ≈0.65–0.80) 11…”

Zinc Complexes of New Chiral Aminophenolate Ligands: Synthesis, Characterization and Reactivity toward Lactide

“…Furthermore, increasing the amount of initiator to 100:1 under the same conditions did not yield any polymer. Due to the small size of zinc metal, in order to effectively carryout chain-growth polymerization of LA via coordination-insertion mechanism, most researchers have been successful at a temperature of 130˚C [88]- [101]. Even though this is not somewhat surprising, it is hypothesized that attaching a larger metal such as calcium or lanthanum to the newly synthesized chiral aminophenolate ligands would provide better catalytic reactivity at milder conditions with better polymerization control.…”

“…With moderate reactivity of zinc allowing for more polymerization control, attaching chiral aminophenolate ancillary ligands with varying substituents and pendant arms may induce high isotactic/heterotactic selectivity in the ROP of rac-lactide. To the best of our knowledge, only Wang and Ma have reported the diastereoselective synthesis of chiral aminophenolate zinc complexes with multiple stereogenic centers and their isoselective polymerization of rac-lactide [101]. Herein we report the synthesis and characterization of new chiral aminophenolate zinc complexes with varying substituents and pendant donor arms containing one stereogenic center, and their reactivity toward racemic lactide.…”

“…Zinc complexes are efficient catalysts for ROP of lactides with moderate reactivities compared to the highly electropositive lanthanide metals and have recently received much attention due to its flexible coordination chemistry, substitutional lability, Lewis acidity and non-toxicity [88]- [101]. With moderate reactivity of zinc allowing for more polymerization control, attaching chiral aminophenolate ancillary ligands with varying substituents and pendant arms may induce high isotactic/heterotactic selectivity in the ROP of rac-lactide.…”

Highly Branched Poly(α-Methylene-γ-Butyrolactone) from Ring-Opening Homopolymerization

“…The best and most reliable method of synthesizing high molecular weight polyesters is via ring-opening polymerization (ROP) of lactones with a relatively high strain energy using metal initiators for chain-growth polymerization [11]- [35].…”